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20-Bit, Single-Channel, Ultra-Low-Power, Delta-

Sigma ADCs with Programmable Gain and GPIO

Figure 8. RTD Temperature Measurement Circuit

Figure 9. Resistive Bridge Measurement Circuit

Applications Information

See Figure 8 for the RTD temperature measurement circuit

and Figure 9 for a resistive bridge measurement circuit.

Adding more active circuitry to the analog input signal

path is not always the best solution to a small-signal

problem. Sometimes, increasing the dynamic range of

an active device can lead to a simpler solution that also

helps power consumption and linearity.

Often, circuit designers immediately look for an exter-

nal op amp or programmable gain amplifier (PGA)

when confronted with coupling low-amplitude signals

to sampled digital systems. In many cases, choosing

an ADC with more dynamic range and better low-noise

performance yields a solution that works better, simpler,

and with less power.

One such example is measurements from a strain gauge

in a Wheatstone bridge configuration. Assuming a dif-

ferential output signal from the bridge in Figure 10, the

bridge’s output voltage varies from 5mV to 105mV,

while the noise of the bridge itself limits the sensitivity

to approximately 1FV. This gives approximately 100,000

discrete levels that are available for quantization, a feat

accomplished quite well with any ADC having 17 bits or

more of usable resolution. However, as it is not likely that

a 21-bit ADC will have an input range of 105mV, a gain

stage is needed to boost the signal to span the input

range of the ADC (typically between 3V and 5V).

This solution requires finding an amplifier and associated

passives that meet the overall system noise and linearity

needs. Also, the power consumed in the gain stage may

equal or surpass that of the ADC itself, a fact that is sig-

nificant in systems where power consumption is severely

constrained, such as portable sensors or 4–20mA loops.

The low-noise floor of the MAX11206 family of 16-/18-/20-

bit devices gives the designer the ability to use simple

binary shifting (digital gain) of the data word to align the

sample range with the available code space. Digital gain

is internally available in the MAX11206.

REFP

REFN

AINP

AINN

GND

MAX11206

MAX11207

REFP

AVDD

REFN

AINP

AINN

MAX11206

MAX11207